1. Field of the Invention
The present invention relates to the encapsulation of thin workpieces, particularly electronic and microelectronic devices, to achieve a very thin, but void-free seal around the workpiece.
2. Description of Related Art
Microelectronic devices must be protected against moisture as well as assembly process and other environmental contaminants. This is commonly done by encapsulating the device in a mold compound, such as a thermosetting plastic, applied by a transfer molding process.
In a typical transfer molding machine used in the microelectronics industry, a thin electronic workpiece mounted on a lead frame is clamped between two halves of a split mold. The mold defines a mold cavity around the device with sufficient clearance to allow mold compound to be injected and flow around the device to encapsulate it. During the molding process mold compound is injected into an inlet and air inside the mold escapes from a vent.
The mold compound is initially provided in a non-liquid pellet form containing a desired quantity of the compound. The pellet is heated under pressure in a chamber until it is liquefied. A plunger then drives the liquefied mold compound into the mold cavity. The mold compound is allowed to cure and the mold is opened, releasing the encapsulated microelectronic device.
Because smaller microelectronic devices are highly desirable, device manufacturers would like to reduce the thickness of the encapsulating layer of mold compound which encases each device. Thinner encapsulating layers also aid in improving device performance or reliability with regard to heat dissipation, resistance to coating damage under thermal stress and other parameters. However, as the distance between the inner mold surfaces and the electronic workpiece is decreased, it becomes more difficult to obtain a high quality void-free encapsulant around the entire device.
To obtain a void-free seal, the liquefied mold compound must enter the mold inlet and entirely fill the space in the mold cavity before the mold compound flow front arrives at the mold vent. If the mold compound reaches the vent before the mold is completely filled, an air bubble is trapped in the mold, creating a void.
To completely fill the mold cavity, the mold compound must flow between the upper mold surface and the upper surface of the device, between the lower mold surface and the lower surface of the device, and into the space surrounding the outer perimeter of the device. However, as the distance between the upper and lower mold surfaces and the device is reduced, so as to make the encapsulating coating thinner, it becomes more difficult for the mold compound to penetrate these regions.
If this distance is reduced too far, the mold compound will flow around the outer perimeter of the device before the mold compound flow front has displaced the air in the space above and below the device. The result is a void in the encapsulation material as an air bubble is pinched off in the center of the device.
As a result, transfer molding of semiconductor devices with conventional equipment has required that the distance from the inner mold surfaces to the device be at least about 200-250 micrometers. This ensures that there will be laminar flow of the molding compound into the mold and around the device. The exact minimum distance limit is, of course, a function of the specific mold compound used, the fillers it contains and process parameters, such as temperature, but, in general, reducing the distance from the inner mold surfaces to the device to less than some minimum distance results in unacceptable manufacturing losses due to the formation of voids.
Provided that sufficient clearance between the inner mold surfaces and the device is maintained, however, the flow of the mold compound during injection remains laminar, and the flow fronts above and below the device remain relatively balanced, so as to prevent the formation of voids. On the other hand, it is known that acceptable sealing of the device and protection against environmental contamination can be achieved with an encapsulation thickness that is well below this thickness limit.